1. Field of the Invention
The invention relates to an active matrix (AM)-type flat panel display including thin film transistors, and more particularly, to a flat panel display including active layers of thin film transistors formed of polycrystalline silicon, whose crystal grains have different sizes and shapes for different display pixels.
2. Description of the Related Art
Thin film transistors (TFTs) are used as switching units to control the operations of pixels or as driving units to drive pixels in flat panel displays, such as, liquid crystal displays (LCDs) or organic or inorganic electro-luminescent (EL) displays.
A thin film transistor generally has a semiconductor active layer, a gate insulating layer formed on the semiconductor active layer, and a gate electrode. The semiconductor active layer is formed on a substrate and has a drain region and a source region which are doped with impurities of a high concentration and a channel region formed between the drain and source regions. The gate electrode is formed on a portion of the gate insulating layer that is over the channel region of the semiconductor active layer. The semiconductor active layer can be either an amorphous silicon layer or a polycrystalline silicon layer.
Thin film transistors using amorphous silicon can be deposited at a low temperature, but have degraded electrical characteristics and low reliability. Thus, thin film transistors using amorphous silicon are not suitable for large-sized display devices. Hence, polycrystalline silicon thin film transistors are more commonly used. Since a polycrystalline silicon has a high mobility of several tens through several hundreds of cm2/V.s, a low high-frequency operating characteristic, and a low current leakage value, the polycrystalline silicon is very suitable for use in large, high-definition flat panel displays.
As described above, thin film transistors are used as switching units or pixel driving units in flat panel displays. An AM-type organic EL display generally includes at least two thin film transistors (hereinafter, referred to as TFTs) for each sub-pixel.
An organic EL element generally has an organic luminescent layer between an anode and a cathode. In the organic EL element, as an anode voltage and a cathode voltage are applied to the anode and the cathode, respectively, holes introduced from the anode are transported to the organic luminescent layer via a hole transport layer, and electrons introduced from the cathode are transported to the organic luminescent layer via an electron transport layer. In the organic luminescent layer, the electrons and the holes are combined to produce exitons. As the excited state of the exitons is changed to a ground state, fluorescent molecules on the organic luminescent layer emit light to form an image. In full-color organic EL displays, pixels emitting three colors, generally, red (R), green (G), and blue (B), are included as the organic EL element to achieve a full-color display.
However, in such organic EL displays, R, G, and B luminescent layers emitting R, G, and B light, respectively, have different luminescent efficiency for colors. When the same current has been applied to each of R, G, and B pixels, some colors have low luminescence and some colors have high luminescence, according to the respective colors luminescent efficiency. Accordingly, achieving a proper color balance or white balance is difficult. For example, if the luminescent efficiency of the G luminescent layer is about 3 to 6 times higher than those of the R and B luminescent layers, a current which is about 3 to 6 times greater than a current for the G luminescent layer should be applied to the R and B luminescent layers in order to adjust the white balance to a proper level.
A conventional method of adjusting the white balance to a proper level as described above is disclosed in Japanese Patent Publication No. 5-107561, wherein different voltages supplied through a driving line, that is, different Vdd values, are applied to different pixels.
Japanese Patent Publication No. 2001-109399 discloses a method of adjusting a white balance by controlling the size of a driving TFT. More Specifically, when the width and length of a channel region in the driving TFT are W and L, respectively, different W/L ratios are applied to R, G, and B sub-pixels to thereby control the amounts of currents flowing into R, G, and B organic EL elements.
Japanese Patent Publication No. 2001-290441 discloses a method of adjusting a white balance by forming pixels of different colors to have different sizes. In this method, a green luminescent region with the highest luminescent efficiency is formed to have the smallest luminescent area compared to red and blue luminescent regions, thus achieving a proper white balance and a display device with a long life span. The difference in the luminescent area can be obtained by varying the anode area.
A method of adjusting luminescence by controlling the amount of current by applying different voltage ranges to R, G, and B color pixels via a data line is also known as a method of adjusting a white balance.
However, methods described above do not consider the crystal structure of a polycrystalline silicon TFT of a flat panel display. In other words, the crystal grains of a TFT active layer can have various shapes and sizes according to a crystallizing way, and the mobility of current may vary according to the shape and size of the crystal grains. In this case, the white balance cannot be adjusted even by using the above-described methods.
If the amount of current flowing into an organic EL element in each sub-pixel exceeds a limit value, luminescence per unit area is greatly increased by the amount of current. Accordingly, the life span of the organic EL elements rapidly decreases. Thus, to increase the life span of organic EL elements, an optimal amount of current has to be supplied to each sub-pixel.